WASHINGTON — Terror suspects who have been held but released from Guantanamo Bay are increasingly returning to the fight against the United States and its allies, the Pentagon said Tuesday.

Sixty-one detainees released from the U.S. Navy base prison in Cuba are believed to have rejoined the fight, said Pentagon spokesman Geoff Morrell, citing data from December. That’s up from 37 as of March 2008, Morrell said.

The new figures come as President-elect Barack Obama prepares to issue an executive order during his first week in office to close the controversial prison. It’s unlikely, however, that the Guantanamo detention facility will be closed anytime soon as Obama weighs what to do with the estimated 250 Al Qaeda, Taliban or other foreign fighter suspects still there.

About 520 Guantanamo detainees have been released from custody or transferred to prisons elsewhere in the world.

Morrell said more than 100 detainees have been transferred or otherwise released over the last year alone.

“There clearly are people who are being held at Guantanamo who are still bent on doing harm to America, Americans, and our allies,” Morrell told reporters at the Pentagon. “So there will have to be some solution for the likes of them, and that is among the thorny issues that the president-elect and his new team are carefully considering.”

Morrell said the new numbers showed a “pretty substantial increase” in detainees returning to terror missions — from 7 percent to 11 percent.

He said intelligence, photographs and forensic evidence like fingerprints and DNA were used to tie the detainees to terror activity. He did not know where they had been released, or what missions they are now believed to have rejoined.

Human rights activists and defense lawyers for the detainees argue that many Guantanamo prisoners pose no security risk and should be released.

In a recent report, the Brookings Institution examined hundreds of pages of declassified military documents, and ultimately said it couldn’t tell whether many of the prisoners held for years without charges are terrorists or innocent.

The Washington think-tank concluded that only 87 of the 250 detainees described having any relationship with Al-Qaeda, the Taliban or other armed groups considered hostile to the United States.